The present invention relates to spectral monitoring instruments and in particular to instruments for monitoring wavelengths of narrow band ultraviolet lasers.
Spectrometers are well known devices for measuring the intensity of light at various wavelengths. A typical spectrometer consists of a slit, a collimator lens, a dispersive optic, such as a prism or grating, an objective lens or lenses for focussing the various wavelengths and a photometer for measuring the intensity of the various wavelengths. FIG. 1A is a schematic drawing of such a prior art grating-based spectrometer. A light source 2 which is the subject of a wavelength measurement is sampled by an optical fiber 4 having an internal diameter of about 250 microns and a portion of the light is directed to slit 6 which is longer than the internal diameter of the fiber and has a width of about 5 microns. Light passing through slit 6 expands in the 5 micron direction in a beam 7 at an angle of about 3 degrees. The beam is reflected from mirror 8 and is collimated by lens 10 for illumination of grating 12 which in this prior art representation is arranged in a Littrow configuration. Light at various wavelengths reflecting from the grating is dispersed at angles dependant on the wavelengths. A beam representing only one wavelength is depicted in FIG. 1 as reflecting from the grating 12 back through lens 10 and reflecting off mirrors 8 and 14 and is focused to a line at 15. (The long dimension of the line is into and out of the page.) This particular wavelength is refocused at a line 17 by objective lens 16. Light at this wavelength is measured by a photometer 18, while light at other wavelengths is blocked by a slit 19 placed in front of the photometer 18. Slit 19 and photometer 18 are placed in the same housing. Light at wavelengths other than the depicted wavelength is reflected off grating 12 at angles slightly different from that of the depicted beam. Thus, other wavelengths are measured at positions above or below line 17 by photometer 18 which, as indicated in FIG. 1, moves back and forth, together with slit 19, to make these intensity measurements.
The resolution of this prior art spectrometer is limited by dispersion of the grating and its size. Both of these parameters can only be improved up to a certain level determined by technology limits and cost. If desired parameters still cannot be achieved, then several diffraction gratings can be used in more elaborate spectrometry. This will proportionally increase the resolution. However, these more elaborate techniques can substantially increase the cost and the size of the spectrometer. What is needed is a simple and inexpensive method of substantially increasing the precision of prior art spectrometers. A particular need exists for a compact, high resolution ultraviolet spectrometer with a resolution of the order of 0.1 pm. Such a spectrometer is needed to monitor the output spectrum of narrow band excimer lasers used, for example, in micro lithography.
It is well known that a Fabry-Perot etalon may also be used as the dispersive element rather than a diffraction grating. Etalons are routinely capable of producing resolving powers on the order of 107. Because etalons do not require the use of a slit aperture, their luminosity is high. Unfortunately, to achieve high resolving powers with an etalon spectrometer one would traditionally have to sacrifice free spectral range.
The transmission of an etalon when illuminated by a diffuse monochromatic source is maximized at specific angles. These fringes of equal inclination produce a concentric ring pattern when imaged by a lens. The angular separation between consecutive fringes of an etalon defines the FSR of the etalon in angle space. The relationship between the maximum angle xcex8, of an etalon with respect to wavelength is defined by:
mxcex=2nd cos(xcex8)xe2x80x83xe2x80x83(1)
where:
m=fringe order
n=index of refraction
d=plate separation of etalon
These multiple fringes or pass bands in the transmission of a single etalon limit its usefulness to a region between consecutive fringes. In a typical etalon spectrometer, the usable spectral range is limited to about 30 to 50 times its resolution. However, in order to measure the spectrum of an excimer laser used for microlithography a much larger spectral range is required.
Line narrowed excimer lasers are currently used as the light source for microlithography. In order to provide integrated circuit feature sizes in the range of a small fraction of a micron, the bandwidth of the laser beam must be narrowed to a fraction of a picometer and the central wavelength must be controllable to an accuracy of a small fraction of a picometer. FIG. 1B is a drawing of a narrow band excimer laser system 1 showing a typical scheme for controlling the wavelength and bandwidth of these excimer lasers. A gain medium is created in laser chamber 22 by electric discharges between two elongated electrodes 24 (only the top electrode is shown). At the rear of the chamber, the laser beam exits into a line narrowing package, LNP, 26 which comprises a three prism beam expander 28, a tuning mirror 30 and a grating 32 arranged in a Littrow configuration. Tuning mirror 30 is arranged to pivot about an axis as indicated in the figure and its position is controlled by a precision driver unit 34 such as a stepper motor or a piezoelectric driver or a combination of the two for wide tuning range and precise control. Precise control is provided in a feedback arrangement in which a portion of the output beam downstream of output coupler 36 is sampled by very fast response wavemeter 3 which measures the central wavelength and bandwidth and controls the central bandwidth to a target value by appropriate feedback signals to driver unit 34.
In order to characterize the spectral properties of microlithography excimer lasers, two specifications are commonly used. The first one is the full width at half maximum (xcex94xcexFWHM), and the second one defines the range containing 95% of the total laser pulse energy. This specification is commonly referred to as xcex94xcex195% and it defines the amount of energy which is contained in the spectrum tails. In the typical microlithography excimer laser, xcex94xcex195% is about three times larger than xcex94xcexFWHM. In order to accurately measure both xcex94xcexFWHM and xcex94xcex195% a spectrometer with resolution of about 0.05 pm and usable spectrum scan range of at least 10 pm is required. These two parameters are extremely difficult to achieve simultaneously using prior art spectrometers. The etalon spectrometer even though capable of providing 0.05 pm resolution will have a usable scanning range limited to 1-2 pm at this resolution. On the other hand, grating spectrometer, having resolution of 0.05 pm at 193 nm is extremely bulky and very expensive device.
What is needed is a comparably inexpensive device which would provide simultaneously resolution of 0.05 pm and scanning range of 10 pm.
The present invention provides a high resolution etalon-grating monochromator. A preferred embodiment presents an extremely narrow slit function in the ultraviolet range and is very useful for measuring bandwidth of narrow band excimer lasers used for integrated circuit lithography. Light from the laser is focused into a diffuser and the diffused light exiting the diffuser illuminates an etalon. A portion of its light exiting the etalon is collected and directed into a slit positioned at a fringe pattern of the etalon. Light passing through the slit is collimated and the collimated light illuminates a grating positioned in an approximately Littrow configuration which disburses the light according to wavelength. A portion of the dispursed light representing the wavelength corresponding to the selected etalon fringe is passed through a second slit and monitored by a light detector. When the etalon and the grating are tuned to the same precise wavelength a slit function is defined which is extremely narrow such as about 0.034 pm (FWHM) and about 0.091 pm (95 percent integral). The bandwidth of a laser beam can be measured very accurately by a directing portion of the laser beam into the monochromator and scanning the laser wavelength over a range which includes the monochromator slit wavelength.